The monitoring of ambient air quality, especially in the workplace, is important not only for complying with legislative permissible levels of various gases or chemical compounds, but also because of the potential health hazards that may be imposed by toxic or flammable gases. As a result, various air quality programs have been utilized for monitoring or analyzing air quality. However, air quality monitoring is only as reliable as the quality of data achieved using currently available air analysis systems, such as Fourier transform infrared (FTIR) gas analysis systems.
Virtually all compounds absorb infrared energy. In FTIR monitoring, infrared energy passes through a sample area, acquiring a characteristic "fingerprint" of the chemicals present due to the unique set of wavelengths they absorb. Currently available FTIR analysis systems for monitoring air quality generally include extractive analysis systems and open air systems. The extractive systems typically consist of a source of mid-infrared radiation, an interferometer, and an enclosed sample cell of known absorption path length, temperature, and pressure. Further, the extractive systems typically include an infrared detector, optical elements for the transfer of infrared radiation between components, and gas flow control and measurement components. Adjunct and integral computer systems and spectroscopic software are used for controlling the FTIR systems, for processing the signals detected by the infrared detector, and for performing both Fourier transforms and quantitative analysis of spectral data. These systems typically monitor many infrared wavelengths simultaneously, and pass on the information detected to the computer system, where it can be transformed into a spectrum. The spectroscopic software analyzes the spectral information. Multi-component analysis of the data can immediately determine which of a certain set of species are present, and how much of each species is present. Further, the spectrum can be analyzed in many cases to determine if any unexpected species were detected; identification of such species can also be made in many cases. The absorption spectrum of pure gases in a mixture of gases are described by a linear absorption theory referred to as Beer's law. Using this law, FTIR systems use the computerized analytical spectroscopic software to quantify compounds by comparing the absorption spectra of known (i.e. reference) gas samples to the absorption spectrum of the sample gas. Such systems normally store the data permanently on a storage media, such as disks, to record the conditions of the site analyzed for use at later times.
Such extractive FTIR systems, available from MIDAC Corp., Costa Mesa, Calif. and others, which use an enclosed sample cell, can be calibrated effectively using known reference gases, i.e. calibration transfer standards, and properly prepared spectral reference data. For example, a calibration transfer standard can be run through the enclosed sample cell resulting in an absorption spectra that, when compared to the standard's known spectra under a different set of conditions pertaining to the spectral reference data, can be utilized to calibrate the FTIR system.
However, unless heated, such extractive FTIR systems utilizing enclosed sample cells cannot always reliably detect various compounds, including many volatile and semi-volatile organic compounds at low concentration levels, such as 10 ppm and below. The sampling and handling utilized with extractive FTIR systems, such as when air is collected as a grab sample at a site to be monitored and then transported to an extractive analysis system, leads to less than desirable quality for the data resulting from the extractive analysis then performed. For example, the grab sample may not be a homogenous representative sample of ambient air at the site being assessed. Further, at low concentrations, both the enclosed sample cell utilized to perform the analysis, and the container utilized for transport of the air sample, can significantly affect the concentration levels of various compounds within such structures. For example, many compounds will react with the wall structures or may stick to wall structures of the enclosed sample cell or transport container such that when analysis is performed, accurate concentration levels are not in the sample path of the system and therefore, not effectively measured. In addition, the time required to obtain results from such a process using a grab sample and off-site analysis is relatively long.
The open air analysis systems available, such as FTIR open air analysis systems, typically include elements like those of the extractive systems but do not utilize an enclosed sample cell. Such open air analysis systems are currently available from MIDAC Corporation, Costa Mesa, Calif. and others. These open air analysis systems, for example, are said to have versatility in that they are able to monitor multiple gas species simultaneously over large sample areas, can provide results in very little time, and are said to be portable and capable of running on battery power in remote locations. However, such systems utilize components that are unconnected physically and located at a substantial distance from one another. For example, such systems may be utilized for fence line monitoring where the interferometer, source, and detector are all located on the ground a substantial distance from reflective elements of the system, i.e. about the perimeter of a particular site. However, because of the large distances between components used to obtain an adequate pathlength for the system, such open air systems cannot be effectively field calibrated, i.e. recording of calibration transfer standard (CTS) spectra in the field. An enclosed sample cell cannot be utilized to encompass such a large sample volume between the components of such an open air system. Therefore, there is no enclosed sample cell into which a calibration transfer standard can be introduced. In addition, the large linear distance between components allows the possibility that the effective sample is not at uniform or nearly uniform temperature. It is clear from these considerations, that the quality and validity of such data collected utilizing such open air monitoring systems is questionable.
For the above reasons and the reasons that will become apparent from the description below, improvements to open air analysis systems and methods for performing such open air analysis are needed. For example, there is a need for open air analysis systems and methods that can perform near real-time on site analysis and have the capability to demonstrate instrument calibration using gas flows of, for example, calibration transfer standards.